Self-protecting electric discharge device



Patented Apr. 18, 1950 SELF-PROTECTING ELECTRIC DISCHARGE DEVICE Daniel Marie Duinker and Johannes Gijsbertus Wilhelm Mulder, Eindhoven, Netherlands, assignors to Hartford NationalBank and Trust Company, Hartford, Conn., as trustee Application July 20, 1946, Serial No. 685,205 In the Netherlands June 28, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires June 28, 1963 4 Claims.

This invention relates to an ionic discharge tube comprising a liquid cathode and a capacitative igniter. A capacitative igniter is formed by a conductor which is separated from the liquid cathode by a layer of insulating material. The ignition is initiated by reason of a spark being struck between the cathode surface and the insulating material due to the high voltage supplied to the conductor. At the level of the cathode surface the conductor is sometimes not in direct contact with the insulating material thus, for example, the conductor may be enclosed within a tube of insulating material which, for example, is filled with a rare gas at a pressure of several millimeters. From their operation, the capacitative igniters are sometimes referred to as spark igniters.

Tubes comprising a liquid cathode are known for their long life and their high overload capacity during a short time. The liquid cathode is not destroyed by heavy currents as in the case of an incandescent cathode. The duration and value of the permissible overload are given by the requirement that not a single member of the tube must acquire a detrimental rise in temperature. In connection with a given type of tube it is often stated how heavily the tube may be overloaded for a given length of time, for example, 100% for seconds, 50% for 5 minutes. If the tube is then overloaded still longer, it will soon become defective as a result of excessive temperature of any member of the tube.

In practice tubes are mostly protected by automata or safety fuses which do not switch off the tube until at several times the normal current in order that load pulses may be absorbed. With reference to the permissible overloads mentioned above by way of example, it will be evident that in this case it is always possible to overload a tube in a detrimental way by adjusting the current to a value much above the normal value but below the value at which the tube is automatically switched off.

An automaton which switches off the tube after the aforesaid overload of so great variety is realized in practice with difficulty. The duration and magnitude of the permissible overload depend in addition on whether a cold tube or a hot tube is started with. As the automaton must be adjusted to the most unfavourable case, it is consequently impossible to have the tube perform its maximum achievement under all conditions.

If the tube is protected by a maximum automaton which even cuts off the tube at a slight higher current than the normal one, it will practically always be necessary to choose the power of the tube materially higher than is required for the mean load, since otherwise load pulses cannot be absorbed.

The invention obviates these disadvantages in that due to suitable construction and proportioning of an ionic discharge tube comprising a liquid cathode and a capacitative igniter, the temperature and the pressure increase to such an extent in the case of overload, that the igniter becomes inoperative before any part of the tube sustains damage by excessive temperature.

Capacitative igniters can no longer start up the discharge beyond a given temperature of the cathode and a given pressure in the tube. For a given igniter, the tube containing only mercury, it has been found that it fails at a cathode temperature of about '80 C. (vapour pressure about 0.1 mm). The failure of the igniter may be accounted for by the fact that in the case of excessive temperature and pressure the charge of the source of voltage may be conducted away from the igniter through a glow discharge between any point (coated or not with insulating material) of the igniter and the cathode instead of through the starting-up spark. The glow discharge is not capable of starting up the discharge in the tube itself, unless abnormally high anode voltages are resorted to.

If therefore the igniter fails, the tube cannot be loaded to a further extent nor can it be damaged by overload. In this case, the connected apparatus, such for instance, as transformers and the loads connected to the tube, if they are altered to accord with the tube, cannot be overloaded either.

For the purpose of proportioning a tube according to the invention the following method may be adopted. First of all it is ascertained at what cathode temperature and pressure the type of igniter to be used fails. With reference to the quantities of heat to be anticipated that are produced in the tube at full load, it is calculated what the size of the cathode and the other tube members should be in order to ensure that the cathode temperature becomes higher than that at which the igniter starts to fail and at which some other tube member just does not assume an inadmissibly high temperature.

It entirely depends on the thermal capacity of the tube whether even transient overloads can be absorbed.

Care should particularly be taken that the cathode does not heat up more slowly than the remainder of the tube, as otherwise certain members of the tube attain an inadmissible temperature even before the igniter becomes inoperative.

Apart from the proportioning of the tube, the latter may be constructed in such manner that supplementary means for ensuring the effect of the invention are arranged on the tube wall. As an alternative, the two measures may be combined.

With the usual tubes having a liquid cathode the cathode is at full load or even in the case of heavy overload generally too cold and the pressure in the tube too low to cause the igniter to fail. A simple means of ensuring the failure of the tube in the case of inadmissible overload is surrounding the cathode space by heat insulating material. In this case the cathode does not cool so intensely and the vapour pressure increases.

It is particularly advantageous to provide means for supplementary heat exchange between the anode and the cathode spaces in a tube according to the invention. Practically always several times more heat is developed at the anode than at the cathode. Conducting away this heat to the liquid cathode causes the temperature of the cathode and the pressure in the tube at the same load to increase to a higher extent than would be the case without this heat exchange.

The establishment of the heat contact between anode and cathode spaces may be effected, in the case of cooled tubes, in a very simple manner by arranging the pipes for the cooling agent to surround the tube in such manner that the cooling agent first cools the anode and then flows about the cathode. As an alternative the whole tube, at least the anode and the cathode spaces, may be placed in a liquid bath which is cooled in a natural or artificial way. The liquid bath sets up a heat exchange between the anode and the cathode, so that the cathode becomes hotter.

In the case of tubes according to the invention, it is particularly advantageous also to liquify the anode, as the intense heating of the anode results in a distillation of the electrode liquid to the cathode. In this case, the pressure above the cathode is higher than corresponds with the cathode temperature and the igniter will be more apt to fail.

A simple means of attaining the object of the invention with tubes comprising two liquid anodes and cathodes, is also to make the cathode several times smaller than the anode. This results in an increase of the cathode temperature and the vapour pressure.

The invention may also be applied to tubes comprising two liquid electrodes each of which is provided with a capacitative igniter. These tubes may serve both for the passage of a rectified alternating current, alternately in one or in the other direction, and for the control of the alternating current supplied to the load.

In order that the invention may be clearly understood and readily carried into eifect, the provision of supplementary means according to the invention will now be described more fully with reference to the accompanying drawing, in which- Figure 1 is a vertical section of a tube according to the invention, in which the cathode space is surrounded by heat insulating material, and

Figure 2 is a vertical section of a tube whose cathode is several times smaller than the anode and in which'the cooling air, after being heated by the anode, brushes past the cathode space.

Referring to Figure 1, I designates the wall of of the tube, 2 the cathode consisting of mercury, 3 the anode formed by a block of graphite, 4 and 5 designate respectively the current leads for the cathode and the anode. The capacitative igniter is constituted by a thinwalled. glass globule 6 immersed in the cathode and partly filled with mercury. A wire 7 leads to the terminal 8 of the igniter. The cathode space is surrounded by heat insulating material 9 so that at full load of the tube the cathode assumes such temperature and the vapour pressure increases to such extent that the igniter just does not fail. Now, if the tube is loaded to an even higher extent, the igniter will no longer be able to start up the tube, so that the tube cannot be damaged by excessive temperature.

In Figure 2, the parts corresponding with those of Figure 1 are designated by like reference numerals as in Figure 1. The current supply leads for cathode and anode are formed by copper strips 4 and 5 which are at right angles to each other so that erroneous mounting of the tube in the socket is impossible.

The anode and cathode spaces are connected by a, comparatively narrow connecting tube ID, in which provision is made for a container II which serves to retain the mercury running over from one Of the electrodes. In the use of the tube the terminals I2 and I3 are connected to a source of very low voltage. The mercury contained in the reservoir I I establishes an electric connection between the terminals l2 and I3 and is then volatilized by electric heating. Consequently the reservoir II will not overflow and the mercury running over from one of the electrodes cannot establish a short circuit between the two electrodes. Surrounding the anode and the cathode spaces is arranged an envelope I4 through which cooling air may be blown. A connecting flange I5 for the cooling air conduit is arranged on the anode side and on the cathode side provision is made for a small diiTuser I6 for conducting away the cooling air. As is clearly shown on the drawing, the cathode, as distinguished from usual tubes, is much smaller than the anode. In spite of the fact that at the cathode less heat is produced than at the anode, the cathode will consequently assume a high temperature even at a normal load of the tube. If the heat produced at the anode increases to a very high extent the cooling air will be intensely heated by the anode and again transmit heat to the cathode which is not heated to such an extent. When proportioning the tube it should be particularly taken into account that the connecting tube I0 must be chosen to be so narrow that no backfiring can occur in the tube and the temperature of the electrode, at which the igniter becomes inoperative must be such that the tube I0 does not yet assume a temperature in excess of its admissible temperature.

What we claim is:

1. An electric discharge tube having a given maximum operating current, comprising an envelope, an anode within the envelope, a liquid cathode within the envelope, an igniter in capacitative relationship with said cathode, and means coupled to said cathode to heat the same to make the igniter effective in initiating a discharge in said tube at current values less than the said given current and to make the igniter inefiective at current values greater than the said given current.

2. An electric discharge tube having a given maximum operating current, comprising an enve-v lope, an anode Within the envelope, a liquid cathode within the envelope, an igniter in capacitative relationship with said cathode, and means coupled to said cathode to heat the same to make the igniter effective in initiating a discharge in said tube at current values less than said given current and to make the igniter ineffective at current values greater than the said given current, said means comprising a heat exchange fluid thermally coupling said anode and said cathode.

3. An electric discharge tube having a given maximum operating current, comprising an envelope, a liquid metal anode within the envelope, a liquid metal cathode within the envelope, an igniter in capacitative relationship with said cathode, and means including the volume of the cathode to regulate the cathode temperature to make the igniter effective in initiating a discharge in said tube at current values less than the said given current and to make the igniter ineffective at current values greater than the said given current.

4. An electric discharge tube having a given maximum operating current, comprising an envelope, a, liquid metal anode within the envelope, a liquid metal cathode within the envelope and having a volume several times smaller than the volume of the anode, an igniter in capacitative relationship with said cathode, and means including the volume of the cathode to regulate the cathode temperature to make the igniter effective in initiating a discharge in said tube at current values less than the said given current and to make the igniter ineffective at current values greater than the said given current.

DANIEL MARIE DUINKER. J OHANNES GIJSBERTUS WILHELM MULDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

